The protozoan parasite Entamoeba histolytica causes an estimated 50 million cases of invasive disease annually. The most common manifestations of amebic infection are colonic disease and liver abscesses. Our goal is to identify novel virulence determinants in E. histolytica with the aim of understanding the molecular basis of amebic pathogenesis. We have previously developed and used microarray technology to identify genes whose expression is restricted to virulent strains and conditions. One such gene, encoding an E. histolytica serine, threonine, isoleucine, rich protein (EhSTIRP) is expressed only in virulent strains and conditions and has very low to no expression in non-virulent strains and conditions. Preliminary data indicate that parasites in which EhSTIRP is downregulated have decreased adhesion and virulence in vitro. Additionally, in EhSTIRP downregulated parasites have reduced expression of cytoskeletal genes. Our hypothesis is that EhSTIRP is a key virulence determinant in E. histolytica with potential roles in parasite adhesion and signaling relevant to amebic pathogenesis. We propose to further characterize the contribution of EhSTIRP to E. histolytica virulence in vitro, determine whether EhSTIRP is necessary for virulence in vivo, identify the localization of EhSTIRP during host invasion, characterize domains of EhSTIRP with relevance to parasite virulence, and identify amebic proteins that interact with EhSTIRP. We will use genetic and biochemical approaches to dissect gene function, live cell imaging to determine EhSTIRP localization during amebic host cell interaction and microarray analysis to define parasite transcriptome changes associated with genetic manipulation of EhSTIRP. These approaches promise to identify the role of EhSTIRP in amebic pathogenesis and dissect the genetic and biochemical network that is associated with EhSTIRP and which regulates virulence in Entamoeba histolytica. PUBLIC HEALTH RELEVANCE: Entamoeba histolytica is an important pathogen and has an impact on human health on a global scale. The main disease manifestations are dysentery and liver abscesses. Although the majority of disease is in developing countries, this parasite can cause infections anywhere that water purification systems get adversely affected. Events such as the Tsunami or upheaval in the political and social infrastructure of a region can allow disease to emerge. We are interested in understanding the molecular mechanisms that the parasite uses to cause disease.